Description of the Related Art
Printers and the like are traditionally connected to personal computers by means of serial data cables. Serial signal protocols, connectors, and cables typically conform with the RS-232-C standard of the Electronic Industries Association (EIA), a standard for the interface between Data Terminal Equipment or DTE (terminals and personal computers and printers) and Data Communications Equipment or DCE (typically modems) employing serial binary interchange (see also the comparable CCITT standard V.24). Traditionally, a 25-pin standardized connector was used, but newer designs utilize connectors having fewer pins which are also standardized. This high level of standardization makes it relatively simple to interconnect any computer to any modem or any printer.
The RS-232-C standard provides for two-way serial communications over two wires, with data flowing out (from the DTE) over a first wire and back (from the DCE) over a second wire. When a "Data Terminal Equipment" (DTE) connects to a "Data Communications Equipment" (DCE), as when a personal computer connects to an external modem, the pin numbers at each end of the standard RS-232-C cable match up properly so that pin 1 on the computer connects to pin 1 on the modem, pin 2 connects to pin 2, pin 3 connects to pin 3, and so on. But when a DTE connects to a DTE, as when a DTE personal computer connects to a DTE printer, the RS-232-C cable must be rewired so that pins 2 and 3 at one end connect respectively to pins 3 and 2 at the other end; and other wires may also have to be cross-connected to insure a proper hardware handshake, in accordance with the standard and the specialized needs of the equipment at either end of the cable. Such a cable is called a "null modem," since it can be thought of as the equivalent of two DCE modems connected by a telephone line and interconnecting the two DTE's.
In the field of label printing, it is often desirable to provide portable bar coded label scanning equipment, keyboards for entering commands, servers for storing pricing and label image data, and printing equipment that can be carried about and used where needed to print new and replacement labels. Typically, this equipment is directly connected together by means of serial cables, connectors, and protocols of the type just described. Such cables can get in the way of hand-held scanning equipment and can become disconnected. Cables are also subject to wear and tear and are expensive to replace. Serial cables cannot be unduly long, so they sometimes require the keyboard and scanning equipment to be located closer to the printing equipment than is often desirable. To switch between multiple printers, cables may have to be disconnected and reconnected, or cumbersome switch boxes must be used, or multiple jacks and cables must be provided, or a local area network or LAN must be set up and managed.
Radio links can be utilized as a replacement for such cables, but such links can be unreliable, since errors can result from static and interference and the like. Some such radio links appear to the computer and printer to be modems, but such links cannot be used without redesign of the related computer and printer equipment, since modems must be sent special codes that initialize the modems, set transmission speeds, and select protocols. Redesign is required to provide these special codes.
Wireless LANs are known which utilize spread spectrum technology to transmit messages among a series of radio links, but such LANS again cannot be used as direct replacements for serial cables without redesigning the related equipment to implement a local area network. Such systems typically utilize complex spread-spectrum technology and are quite expensive. See, for example, U.S. Pat. No. 5,107,100 which issued on Apr. 21, 1992 to Shepard, et al. (See col.7, line 35 to col. 8, line 13). Spread spectrum technology is explained in U.S. Pat. No. 5,029,183 which issued on Jul. 2, 1991 to LaRoy Tymes.
U.S. patent application Ser. No. 08/502,612, filed on Jul. 14, 1995 by Kelly L. Johnson and Mark W. Roth (and assigned to the same assignee as the present application), teaches use of a single radio link driving a remotely-controllable multiplexer (44 in FIG. 2) to connect any one of multiple printers back to a host computer source of data. Special addressed data packets, sent over the radio link by the host computer, cause the remote multiplexer to switch the radio link to any desired one of several remote printers whose data input cables are plugged into the multiplexer. Once properly switched, the radio link and multiplexer together provide a direct path from the host computer to a particular one of several printers. But the host computer must be specially programmed to know the printer addresses and to send out the special addressed data packets to the multiplexer, and the printers must be plugged directly into the multiplexer. This application teaches the use of data packets similar to those used in the present invention, but their use is not transparent to the host computer which must be especially programmed to generate such packets and to know the addresses of the printers.
Accordingly, existing equipment presently in the field and designed to support conventional serial communication over conventional RS-232-C cables cannot be simply retrofitted to function in the manner just described without radical redesign. Only customized printers and the like can be supported by such systems.